Partial discharge (PD) measurement is necessary for insulation diagnosis and usually a required item of the maintenance procedure for most high-voltage equipment assets. Optical fiber sensor (OFS) provides an innovative and nondestructive approach to convert PD-induced acoustic signal into optical parameter, but the optical probe is challengeable since it is the key component to pick up the extremely weak PD-induced acoustic signal. A universal 3-D skeleton-type fiber probe is designed. To optimize the specific parameters of the optical probe, the multiparameter sensitivity model of tiny skeleton-type optical fiber probe is proposed and established, involving in the material, physical height, radius, and the sensing fiber length. A skeleton-type probe with a wingding length of 60 m, the skeleton height of 12 mm, and peek material is designed for PD ultrasonic sensing. Theoretical modeling, mathematical simulation, sensitivity calibration and comparative PD tests with regard to the sensing probe based the specific optical PD measurement system are carried out. PD results show that the average detection amplitude of the developed OFS is 1.27 times that of commercial piezoelectric transducer (PZT). The flexible installation and the enhanced sensitivity make the Sagnac ultrasonic sensing system an excellent approach to detect and locate weak PD signals inside the power apparatus.